Flexible And Porous Carbon Nanofibers Based On Polypyrrolone: Preparation And Their Charge Storage Performance

Supercapacitor is a new type of energy storage device between the traditional capacitor and the secondary battery.Due to the advantages like high power density,fast charging and discharging speed,high coulombic efficiency and long cycle life,the supercapacitor is widely applied in many fields like transportation,new energy power generation,communications and military project and so on.With the rapid development of the society,lightweight,flexible and wearble,even foldable electronic products have attracted scientists' attention and have been flourishing.As the matching power supply of these flexible electronic products,the supercapacitors tend to be flexible,lightweight,environment friendly and high performance.Therefore,the development of flexible electrode materials with high-performance has important practical significance.In this paper,flexible polypyrrolone(BBB)/polyimide(PI)composite-based carbon nanofibers(PBPICF)with abundant micropore were prepared by carbonizing the electrospun Polypyrrolone(BBB)/Polyimide(PI)composite nanofibers;the high conductive PBPICFs were prepared by mixing carbon nanotubes(CNTs)in the electrospun polypyrrolone(BBB)/polyimide(PI)composite nanofibers and the mesoporous PBPICFs were prepared by mixing polystyrene(PS)microspheres in the electrospun polypyrrolone(BBB)/polyimide(PI)composite nanofibers.The as-prepared PBPICF membranes,which can be arbitrarily bending and folding,are self-standing,fluffy and porous;and possess three-dimensional network structure,good mechanical performance and flexibility.Such PBPICF membranes are directly used as electrode materials without any binder for supercpapacitor.The work on this thesis mainly includes the following three parts:1.Synthesis and Characterization of the flexible polypyrrolone(BBB)/polyimide(PI)composite-based carbon nanofibers(PBPICF): the PBPICFs were prepared by carbonizing the electrospun polypyrrolone(BBB)/polyimide(PI)composite nanofibers.By changing the ratio of polycarboxylic acid ammonium salt(PCAAS)and polyamic acid(PAA),a series of PBPICF membranes were obtained with different ratios of the PCAAS/PAA.The results of electrochemical tests showed that the PBPICF membrane with the ratio of PCAAS/PAA=65/35(marked as PBPICF-65-35)had the best charge storage performance.The specific capacitancereached 172.44 F/g at the current density of 0.2 A/g.After 10000 cycles at 10 A/g,the specific capacitance only decreased by 25%.2.Synthesis and Characterization of the high conductive PBPICF/carbon nanotubes composite nanofibers(marked as PBPICFs/CNTs): the polymer matrix was selected based on the best result of the above part 1 to have the ratio of PCAAS/PAA=65/35.Then,a series of carbon composite nanofibers(marked as PBPICF/CNTs)with different contents of CNTs were prepared by mixing conductive fillers of CNTs in the precursor nanofibers,BBB/PI composite nanofibers.The results of electrochemical tests showed that when CNTs content was 15%,the carbon composite nanofibers(PBPICF/CNTs-15)had the best electrochemical performance.The specific capapcitance reached 194.00 F/g at the charge-discharge current density of 0.2 A/g.When the current density increased to 100 A/g,the specific capaitance was still remained 37%(71.67 F/g),which was the double of pure PBPICF's(18%).Besides,the cycle performance of PBPICF/CNTs-15 was also improved.After 10000 cycles at 10 A/g,its retention of specific capacitance increased from 75% of PBPICF to 80%.The improvement of charge store performance of PBPICF/CNTs-15 is mainly attributed to the incorporation of CNTs,which form conductive networks in the carbon composite nanofibers and provided shortcuts for the electrons to transfer inside the composite nanofibers.As a result,the overall electrode resistance reduced and mesporous volume increased due to the large mespore volume of CNTs.All of these were in favor of the migration and diffusion of electrolyte ions in the electrolyte.3.Synthesis and Characterization of the mesoporous PBPICFs: based on the work in Part 1,the ratio of PCAAS/PAA=65/35 is still selected for the synthesis of mesoporous PBPICFs.Then,a series of mesoporous PBPICFs were prepared by mixing polystyrene(PS)microspheres and polyvinylpyrrolidone(PVP)in the precursor nanofibers,PCAAS/PAA nanofibers.Here,the PS microspheres are used as pore forming agent and PVP as spinning agent.The results showed that the mesoporous carbon nanofiber(PBPICF/PS-30),at a 30% content of PS micropheres,had the best electrochemical performance.The specific capacitance reaches a high value of 214.22 F/g at 0.2 A/g,and still remains 80.4 F/g even at the current density of100 A/g.After 10000 cycles at 20 A/g,the specific capacitance of all PBPICF/PS electrodes still remained 90%,indicating an excellent cycling performance.The improvement were mainly attributed to large amount of mesopores formed by pyrolysis of PS micropheres in the carbon nanofibers.The mesopore structure provides many “highways” and attachment sites for ions in the electrolyte andfacilitates the rapid migration of ions even at a high charge-discharge current density and the storage of more charges.